The most comprehensive source for climate data is NCDC. They charge for custom requests (even for education), but there is a lot you can get to for free when accessing their site from an .edu domain. Help for navigating their site is below, as well as instructions for another useful source, the PRISM dataset from Oregon State/NRCS.

Climate data is tricky because there is so much of it and different stations record different elements and report at different intervals. Also weather stations move over time, which can be a problem for local level anaylses. It is best to verify the location coordinates of weather stations before performing local-level analyses by checking them in Google Earth using address information or other clues.

Depending on your needs, the state climate offices and the Regional Climate Centers may be the best places to go for data. They are usually small operations so often you can actually speak with a climatologist to request what you need, and they are familiar with the data and can best assist you. Also, there may be smaller networks of weather stations in the state or region whose data is not collected by NCDC. They may or may not charge for data.

Using the National Climatic Data Center Website

Below are instructions from Amanda Henley to how to navigate/download data from the NCDC website. The exact steps are subject to change over time, however.

The NCDC has a mapping website at http://gis.ncdc.noaa.gov. This site provides an online map viewer which enables data selection, and provides Web Mapping Services, metadata searching, and KMZ files for certain data.

- To get data, access the system through
this web page.
- Click on "Climate Data Online System (CDO)"
- Under Search Options - Country -> United States.
- Most people will want either the Surface Data, Daily or the Surface Daily, Monthly.
- Select one and click access Data/Products
- Now the steps from here will be different, but basically...

- Select the station(s).
There are several ways to search for the weather station you want. For most people the best way to go would be for the entire state or for a county, or all available stations, but with this option you can get one year of data at a time only. Just note that the best stations- those with the longest periods of record and least missing data- are at airports. Also, the airport stations are typically the ones with hourly observations and more detailed data like wind.

- Select the meteorological elements, date range, and output (comma delimited is recommended).
- Check to make sure the station you've selected has data for the date range you've chosen (station info is near the bottom of the page) and click the box beside Inventory Review.
- Provide an email address. NCDC will email you when the data file is ready for download.
- Click Submit Request and you're done.
- You should be able to bring the files into Excel, as they are comma delimited.

When you first look at the files it is overwhelming, but the file layout is explained in the soddoc.txt file. The station information is included in the ***stn.txt file, and should have a station id, lat/long and elevation. The lat/long usually has just degrees and minutes, and will need to be converted into decimal degrees and then joined to the weather data file using the station id as a unique id.
The datum for this data is unclear, but likely is WGS84. Once the lat/long is joined to the weather data you can create a shapefile of the weather stations and then interpolate to raster. Note that the files created from CDO will be deleted in 7 days.

Using the PRISM Dataset from Oregon State

Below are instructions from Amanda Henley to how to download and use data from the PRISM Dataset. The exact steps are subject to change over time, however.

- Read about the PRISM Dataset here. PRISM data, in a nutshell, is used to estimate climate conditions in mountainous or rural areas where there are few reporting stations. The data format is an interpolated grid which can be used in GIS, based on "...a coordinated set of rules, decisions and calculations designed to mimic the decision-making process an expert climatologist would invoke when creating a climate map."

- At the top of the next page, choose the product you want (Precipitation, Avg Max, Avg Min, etc).
- At the top of the table, Switch to Gridded Data. You will get a matrix of data available (green balls indicate final data).
- Click on one of the green balls. You will get a graphic of the data.
- At the top of that page, click the link "Download Gridded Data".
- After you download it you will need to unzip it.

- Next, you will need to rename the file, as per these instructions. In summary, if you download a file us_ppt_1971_2000.01, rename it to ppt_01.txt or ppt_01.asc). Raster file names cannot be long or have spaces. Also don't store raster data in a path that has spaces (like ...\documents and settings\...).

- Use the import to raster tool in arctoolbox: ArcToolbox -> Conversion Tools -> To Raster -> ASCII to Raster. You want to select float, because with an integer type, "you cannot have fractions or numbers to the right of the decimal place. To store data with decimal values, you will need to use either a float or a double" (from ArcGIS help). You will definitely need numbers to the right of the decimal with precip data.

- According to the metadata, the grids are in a Geographic Coordinate System, World Geodetic Spheroid 1972 (WGS72). Once you get it imported to a raster, you'll need to define the coordinate system. The easiest way to do this is in ArcCatalog. In the catalog tree on the left, navigate to the folder where your new raster lives and select it. The contents of that folder will be listed to the right (use the contents tab). Right-click on your new raster and select Properties. Scroll down, you will see Spatial Reference "Undefined". Click the Edit... button. Define the coordinate system interactively as Geographic, Units DD, Datum WGS 1972.

- Now your data is ready. You can clip it and project just the area you need.

Other Selected Data Resources

The following are selected NOAA weather & climate data resources available on the Libraries Geodata server and CD-ROM: